1. Field of the Invention
The present invention relates generally to Magnetic Resonance Imaging (MRI), and more particularly, to reducing distortion in Magnetic Resonance Imaging (MRI) images of a recipient caused by the presence of a implantable medical device in the recipient.
2. Related Art
Various implants can be implanted into a recipient to provide a medical benefit. Such implants may range from, on the one hand, passive devices such as screws that are utilized to adhere bones together to, on the other hand, active implants, such as, for example, hearing prostheses such as cochlear implants that perform an active function such as stimulating the cochlea of a recipient to enhance hearing. Other exemplary active devices include devices for regulating hormones or regulating cardiac activity.
A range of active implantable medical devices apply electrical energy to tissue of a recipient to stimulate that tissue. Examples of such implants include pacemakers, auditory brain stem implants (ABI), functional electrical stimulators and cochlear implants. Other implantable medical devices perform other functions such as monitoring various functions of the recipient's body such as glucose level, and others can be used to administer drugs internally.
A cochlear implant allows for electrical stimulating signals to be applied to the auditory nerve of a recipient, causing the brain to perceive a hearing sensation approximating the natural hearing sensation, thereby enhancing hearing. These stimulating signals may be applied by an array of electrodes implanted into the recipient's cochlea.
The electrode array is connected to a stimulator that generates the electrical signals for delivery to the electrode array. The stimulator in turn is operationally connected to a sound processor (which may be implanted in the recipient, or which may be located externally to the recipient, communication with the stimulator being via a transcutaneous inductance link) which is in signal communication with a microphone. The microphone receives audio signals from the environment. The sound processor processes these signals to generate control signals for the stimulator.
It is not uncommon for a recipient of an implantable medical device to undergo subsequent medical treatment or investigations related to the implantable medical device or the recipient's condition. Certain post-operative investigations utilize Magnetic Resonance Imaging (MRI).
MRI relies on the application of a strong magnetic field to a recipient's body to generate images of tissue and bone structure. The magnetic field aligns protons (hydrogen atoms) within the recipient's body. These atoms are then be excited into resonance by an applied RF field. When the RF field is removed, the atoms release energy as they exit their excited state. This release of energy is detected by a receiver and utilized to create an MRI image.
The presence of materials within the applied magnetic field which have a magnetic property may distort the magnetic field, causing artifacts in the resulting MRI image. Such artifacts may obscure features, resulting in misdiagnosis of the patients' conditions such as, for example, tumors, that are not able to be resolved in the image due to the distortion.
For example, a cochlear implant may contain a transformer which includes a small quantity of ferromagnetic material. Other materials such as the housing and/or the electronics of the stimulator/receiver of the cochlear implant have an adverse effect.
The human body has a magnetic permeability similar to that of water. Implantable medical devices such as cochlear implants typically have components with permeability different to that of water (e.g., transformer cores, housings, etc.). In the past, designers of implantable medical devices have attempted to address this problem by designing the medical device implant with a minimum of ferromagnetic materials. Alternatively, designers utilized additional components made from materials having low magnetic permeability to counterbalance components made from materials having high magnetic permeability. Some designs rely upon devices having ferromagnetic components that are removable through a minor medical procedure prior to undergoing an MRI scan.